A serial printer for use with a computer or a word processor has incorporated therein a reversible d.c. servomotor to drive the carriage of the printer. As well known in the art, such a servomotor operates to drive the carriage for movement (spacing movement) from one printing position to another, for movement (carriage return movement) from an end-of-the-line position to a preset left-margin position, and for movement (tabulating movement) from a preset position to a tabulation stop position. The latter two modes of movement of the printer carriage require the servomotor to operate at far higher speeds than the speed required for the spacing movement of the carriage. In some serial printers, the carriage drive servomotors are required to operate for carriage return or tabulating movement at speeds which are two to six times higher than the maximum speeds of operation of the servomotors for the spacing movement of the carriages. The spacing movement of the carriage as herein mentioned is the movement of the carriage from one letter to another, from a letter to a space between words, or from a space between words to a letter.
The source voltage, V.sub.S, for the servomotor of a serial printer is generally given as EQU V.sub.S &gt;(.omega..sub.max .times.K.sub.E)+(I.sub.M(max) .times.R.sub.a)+V.sub.CE(sat) Exp. 1
where .omega..sub.max represents the maximum angular velocity (rad/sec) of the motor output shaft as required for the carriage return or tabulating movement of the printer carriage, K.sub.E represents the inductive voltage coefficient (volt/rad/sec), I.sub.M(max) represents the maximum current supplied to the servomotor, R.sub.a represents the resistance between the terminals of the servomotor, and V.sub.CE(sat) represents the saturation voltage between the collector and emitter of the motor drive power transistor. As will be seen from Exp. 1, the higher the the maximum angular velocity .sub.max required of the motor output shaft, the higher source voltage is required for the motor.
Assuming, on the other hand, that the servomotor is to be accelerated with the supply of a constant current, the total power consumption P of the motor drive power transistor during spacing movement of the printer carriage is given by the following expression: ##EQU1## where I.sub.a represents the constant current supplied to the motor under acceleration, t.sub.1 represents the period of time for which the motor is accelerated, t.sub.2 represents the period of time of for which the motor is decelerated until the motor is brought to a stop, .omega..sub.1 represents the angular velocity of the motor output shaft being accelerated, .omega..sub.2 represents the angular velocity of the motor output shaft being decelerated, and I.sub.d represents the mean value of the current supplied to the motor during deceleration. Here, it may be noted that the current I.sub.a during acceleration is larger than the current I.sub.d during deceleration since ordinarily the rate of deceleration is approximately one half of the rate of acceleration and since the motor is subjected to not only an inertial load but a friction torque during acceleration and deceleration. From Exp. 2 it will be seen that the higher the source voltage V.sub.S and/or the lower the angular velocity .omega..sub.1 of the motor output shaft under deceleration, the larger the total power consumption of the motor drive power transistor.
Thus, in order that a servomotor to drive the carriage of a serial printer be enabled to produce a revolution speed required for the carriage return or tabulating movement of the carriage, it is desirable that the servomotor be supplied with a voltage which is high enough to satisfy the relation of Exp. 1. Increasing the source voltage for the servomotor to such an extent however gives rise to an increase in the total power consumption of the motor drive power transistor during spacing movement of the carriage when the printer carriage is driven at relatively low speed, as will be seen from Exp. 2. The increase in the total power consumption in turn might invite an overheat of the transistor and, to preclude an occurrence of such an accident, provision of extra cooling means would be required for the power transistor. Provision of such extra cooling means would result in increases in the production cost, weight and size of the serial printer as a whole. The heat generated from the motor drive power transistor would cause deterioration of the performance reliability of the printer.
As means to enable a motor to operate at an increased speed without increasing the power consumption of a motor drive power transistor, a chopper switch of the "H" type is used which is adapted to chop the base current of the motor drive power transistor to turn on and off the current to be supplied to the motor. One of the problems encountered by the use of such a switch circuit is that sound noises are produced by the switching actions of the chopper switch which occur at frequencies mostly falling within the audible range. Another problem is the generation of disturbances which are produced by the repeated cutoffs of the motor drive power transistor and which are liable to cause errors in the operation of the circuits and circuit elements operatively associated with the power transistor.
It is, accordingly, an important object of the present invention to provide an improved servomotor drive system capable of driving a servomotor without causing the motor drive transistor to consume an unduly large amount of power especially when the servomotor is required to operate at relatively low speeds, although the servomotor is energized with a sufficiently high voltage when required to operate at increased speeds.
It is another important object of the present invention to provide an improved servomotor drive system which is constructed and arranged so that the motor drive transistor forming part of the system is prevented from being unduly heated especially when the servomotor is required to operate at relatively low speeds.
It is still another important object of the present invention to provide an improved motor drive system capable of energizing a servomotor from a high voltage source when the servomotor is required to operate at relatively high speeds and from a low voltage source when the servomotor is required to operate at relatively low speeds.
Yet, it is another important object of the present invention to provide an improved servomotor drive system which is low in production cost, simple and compact in construction, reliable in performance and silent in operation.